Introduction

The most familiar sound birds make is song, which is the often complex series of notes that birds make to establish territory and attract mates. Songs can be sub-divided into categories or functions such as flight, dawn, territorial, courtship etc.
In most North American songbirds, males learn their songs early in life, by listening to their father or local males. They may then sing that song(s) for the rest of their lives, or exchange them for the songs used by the other birds around them when they set up territory elsewhere. Young birds babble at first (known as subsong), then introduce imitations of neighboring birds (known as plastic song), sometimes not perfecting the song until the next spring when they arrive on territory, so many migrants moving north in the spring will not have a crystallized song. In fact they may need to learn new versions from their neighbors when they arrive on the breeding grounds.
Some birds sing only one song, for example the White-crowned Sparrow or Common Yellowthroat, in which case neighboring birds may (usually resident populations) or may not (usually migrant populations) sing the same song. Other birds improvise and create their own unique songs, such as the American Robin. Many species have a repertoire of several songs, and a few sing hundreds, even thousands of different versions of their song, the champion being the Brown Thrasher. Some birds, such as several species of warbler, have two different song types, one to hold territory, the other to attract or keep mates. A few birds continue to expand their repertoire throughout life, such as the Northern Mockingbird. In some species, the female also sings, such as the Northern Cardinal. Others duet, such as the male and female Carolina Wren. A few North American songbirds, the flycatchers, do not learn their songs, they are innate.
Calls are the other vocal sounds birds make, generally short buzzes, chips etc. Examples are flight, alarm, distress, begging, flock, chase, territorial etc. The term call generally denotes sounds other than song, but sometimes the differentiation is indistinct. For example, long sequences of calls by Northern Rough-winged Swallows and House Sparrows and many of the so-called calls by male Red-winged Blackbirds appear to serve the same function as song. In these cases the behavior of the birds, such as sustained calling from a song perch (the sparrow and blackbird) or during a repeated flight circuit before dawn (the swallow) helps to determine the context of the vocalization. The classification of calls can be a little confusing, as calls labeled as flight calls are also regularly given by perched birds, and while alarm calls are generally considered to be made by an agitated bird, in for example the Yellow-rumped Warbler, these calls are also used as flight calls. Perhaps it is best to assume that each call may have more than one function. Conversely, more than one call may have the same function, for example Horned Larks have several different flight calls. Some birds have no known flight calls, such as the vireos. Most calls are thought to be innate, not learnt, and so are less variable than song, and the majority are species-specific, a great way to locate unseen birds or identify non-singing species that are difficult to separate visually, such as the Empidonax flycatchers. While specific calls are much less variable than song, they may vary according to the level of agitation of the bird, from one geographical region to another, or due to environmental factors. Some calls are learned, such as those shared by breeding pairs or migrant groups of cardueline finches.
There are also non-vocal sounds, such as the winnowing of snipe using the outer tail feathers, drumming by woodpeckers, the whistling wing sounds of Common Goldeneye etc.